Nuclear gamma-ray spectroscopy is a technique used to analyze nuclear radiation detected in a radiation detector. When doing a spectral analysis, the nuclear radiation is detected using a nuclear detector. The detector information is recorded into a spectrum, which is a histogram of the different nuclear energies recorded and their rate of detection. The analysis of the gamma-ray spectrum makes it possible to identify the composition of a sample being studied and to determine the quantity of the different elements that compose the sample.
Nuclear gamma-ray spectroscopy techniques are used in many fields including oilfield exploration applications such as wireline logging, logging while drilling, and the like. In oilfield applications, the nuclear spectroscopy technique is for example used to analyze the geochemical composition of a formation around a well for evaluation in the search for hydrocarbons.
One type of radiation detector commonly used for nuclear spectroscopy is a scintillation detector comprised of a scintillating crystal and a photomultiplier tube (PMT) or other device suitable for converting the scintillator light signal into an electric signal. The scintillating crystal is a material that has the property to convert nuclear radiation into optical radiation, or light, that has a wavelength to which the photomultiplier tube is sensitive.
The total signal, i.e. in the case of a scintillation detector the total number of photons, is a function of the amount of energy deposited by the nuclear radiation and of the photon conversion efficiency of the scintillator. This combined function is the response function. For good spectroscopy performance it is essential that the conversion ratio of deposited energy to the number of photons be independent or almost independent of the deposited energy.
An important factor in the quality of the nuclear spectroscopy analysis is the ability of the scintillation crystal to produce consistently the same quantity of light for the same amount of deposited energy. In theory, the detector response to a single energy of nuclear radiation is driven by statistical processes in the energy conversion from photons to electrons and can be approximated by a Gaussian spectral shape; the narrower the Gaussian width, the higher the quality of the spectral data. The width of the peak is quantified by a parameter called “resolution:” for a given scintillator material the better the resolution, the higher the quality of the detector. In theory, assuming that the light output is proportional to the deposited energy, the width of the Gaussian, or resolution, varies with the square root of the energy deposited by the nuclear particles being detected. In reality, the detector's single energy response is not always Gaussian and the width of the response does not always follow the square root of energy. E.g. resolution may depend on the location of impact of the incoming gamma-ray in the detector volume, as either different amounts of light are created in different parts of the crystal or the light collection varies from location to location. Such imperfect behavior is detrimental to the quality of the data obtained with nuclear spectroscopy.
Scintillation crystal compensation is the process where the crystal surface is modified to improve the response function of a detector. This process is widely known in its basic form. Saint Gobain Crystals and Detectors publish the fact (Technical information note Document #526) that the surface of their scintillator crystals is roughened on all surfaces except the surface coupled to the PMT to avoid trapping light in the crystal through total reflection.
U.S. Pat. No. 5,866,908 describes how the reflector properties for individual crystals in scintillator arrays can be modified to obtain a more uniform output level throughout the sensor. Thus, the prior art describes methods to affect detector response level, but not the shape of the detector spectral response or the behavior of the detector response throughout a range of nuclear energy.
Nuclear gamma-ray spectroscopy is used currently in at least three oilfield tools marketed by Schlumberger and other companies are starting to produce their own tools as well. The Schlumberger Wireline Reservoir Saturation Tool (RST) acquires gamma-ray spectra from neutron interactions to produce its answer products. The Wireline Elemental Capture Spectroscopy sonde (ECS) is another tool that analyzes gamma-ray spectroscopy data from neutron interactions. Finally, the EcoScope™ tool provides gamma-ray spectroscopy answer products in logging while drilling. All of these tools' performance is negatively affected when the detector response is not Gaussian and when the resolution does not vary like the theoretical square root of energy.